The present invention relates in general to power supplies and more specifically to a switching power supply.
In the field of IP telephony equipment, an IP phone can be powered by a fixed-frequency, switching power supply employing an industry-standard discontinuous mode flyback topology. With this topology, the power supply output voltage is regulated by controlling the pulse width of the switching waveform in response to changes in both a source voltage and a power supply load. The pulse width control method is commonly known as pulse-width modulation (PWM). Inherent with this method is a potential for very narrow pulse widths at a maximum input voltage and a minimum load when the input operating voltage range is wide. However, the problem with narrow pulse widths is that integrated power supply controllers have a certain propagation delay from their control inputs to their power switch control outputs and the power switch also suffers from its own delay. These delays can become significant at narrow pulse widths particularly when the switching frequency is high and, as a result, the period of each cycle is relatively short.
There are other problems which arise from propagation delays on narrow pulse widths. Firstly, output voltage may be difficult to regulate at high input voltage and light load. Also, a peak primary current limit threshold that is suitable for the rated load at the minimum input voltage results in an excessive output load current at the maximum input voltage.
There are also problems that result from a wide input operating voltage range. One problem is that the blocking voltage rating of power supply output diodes may need to be high which preclude the use of Schottky diodes. Schottky diodes are beneficial for use at low output voltages due to their forward voltage drop, which is lower than standard fast-recovery diodes. This lower voltage drop results in a more efficient power supply, however, it also results in a lower reverse voltage rating. Standard Schottky diodes tend to have a maximum reverse voltage rating of 40V. Another problem is that the switching device must have both a high voltage rating, for operation at the high end of an input voltage range, and a high current rating, to conduct the larger currents associated with operation at a low end of the input voltage range. For efficient power supply operation this combination of high voltage and high current rating in a single device may necessitate the use of a physically larger and more expensive component than would otherwise be required if the input voltage range were narrower.
Presently, with respect to IP phone applications, the power supply is required to operate from two independent voltage sources, VSL and VSH, which have distinctly different voltage ranges. VSL provides a voltage range from 8VDC to 22VDC while VSH provides a voltage range from 22VDC to 56VDC. These voltage ranges result in operation of the power supply over a source voltage range from about 8VDC to 56VDC. For a given power supply load, this 7:1 range in input voltage results in a 7:1 range of PWM pulse width. The 7:1 PWM pulse width ratio, in turn, results in the problems associated with power supply operation using narrow pulse widths and wide input operating voltage range described above.
Prior art techniques have combined the two voltage sources together through coupling diodes to form a single voltage bus having an operating range spanning that of the two sources combined, which in this case would be 8VDC to 56VDC. A wide input voltage range power converter converts the bus voltage to the voltage required by the load. This has been implemented with standard power converter topologies such as the buck converter and the flyback converter. However, this technique still suffers from the problems described above.
The present invention is a switching power supply that improves light load regulation at a high input voltage by doubling the pulse width, thereby overcoming, the problems of in the prior art. The present invention also improves the current limit performance at high input voltage, provides lower reverse voltages for an output diode and optimizes the use of switching devices.
The apparatus of the present invention comprises two distinct voltage buses and an isolating transformer having two primary windings, each with its own associated switching device. Each winding is fed from its own voltage bus with the number of turns on each winding chosen to be proportional to the magnitude of its particular voltage bus. In this way, at any given time, power is supplied from whichever of the two buses is proportionately higher in voltage. The power supply operates from either voltage source alone, or with both sources present simultaneously, with transitions between sources being transparent to the output.
By keeping the two voltage buses separate, the duty cycle range for a given load in this phone application is reduced to 2.75:1 for the low voltage bus and an even lower 2.55:1 for the high voltage bus. This results in the minimum pulse width being over twice as wide as that associated with the 7:1 duty cycle range. This doubling of the pulse width significantly reduces the impact of controller and switching device delays and thus significantly improves light load regulation at high input voltage.
Also, the reduction of the impact of controller and switching device delays improves the current limit performance at high input voltage.
Another advantage of the present invention is that utilization of two, narrow, input voltage ranges allow the transformer turns ratios to be adjusted such that the output diode is subjected to a lower reverse voltage. This allows the use of Schottky diodes for output voltage rails of up to 5V. The benefit of using a Schottky diode is lower power dissipation and reduced component stress.
Finally, having two switching devices allows each device to be chosen so that its parameters are optimized for its particular operating conditions. For example, the low voltage bus device may conduct a high current without having to withstand a high voltage. The opposite is true for the high voltage bus device. This aligns well with switching device technology where the most easily fabricated, and therefore less expensive, devices optimize one parameter, either voltage or current, at the expense of the other. Thus two smaller devices, each optimized for their particular operating conditions, can replace one physically larger, more expensive, device.
According to an aspect of the present invention, there is provided a power supply having an output voltage, comprising: a first voltage source supplying a first DC voltage that is switched on and off by a first switch; a second voltage source supplying a second DC voltage that is switched on and off by a second switch; a transformer comprising a first primary winding connected to the first voltage source, a second primary winding connected to the second voltage source, and a secondary winding, where the secondary winding has an output for supplying the output voltage and where the first primary winding to the second primary winding has a turns ratio that is proportional to a voltage ratio of the first voltage source to the second voltage source, and a pulse-width modulator for switching the first switch and the second switch on and off at a duty cycle to control the output voltage.
According to another aspect of the present invention, there is provided A power supply having an output voltage, comprising: at least three voltage sources, each voltage source supplying a DC voltage and a current that is switched on and off by a switch; a transformer comprising a primary winding connected to each of the voltage sources, and a secondary winding, where the secondary winding has an output for supplying the output voltage and where the primary windings have turns ratios that are proportional to voltage ratios of the voltage sources; and a pulse-width modulator for switching the switch of each of the voltage sources on and off at a duty cycle to control the output voltage.